Design, synthesis and biological evaluation of ansamycin prodrugs as water-soluble Hsp90 inhibitors

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Heat-shock protein 90 (Hsp90) is an ATP-dependent molecular chaperone that is required for stability and function of a large number of proteins, many of which are necessary for cancer cell growth and survival. Inhibition of Hsp90 causes dysfunction and degradation of these client proteins and therefore tumor inhibition.

17-Allylaminogeldanamycin (17-AAG) is the most advanced Hsp90 inhibitor in clinical trials. It is a semi-synthetic derivative of geldanamycin with an improved therapeutic window. However the compound has extremely low water solubility and particularly problematic formulation which brings additional complications. Further analogues such as 17-DMAG, which carries an ionisable dimethylaminoethyl moiety and the hydroquinone 17-AAG (IPI-504) have improved water solubility.

We are interested in developing water-soluble, non-geldanamycin based polyketides as potent Hsp90 inhibitors, and have previously characterised macbecin I as an alternative lead to geldanamycin. Macbecin I has a higher affinity than geldanamycin (Kd 0.24 μM vs 1.2 μM by ITC) for binding to Hsp90 and is more water-soluble than geldanamycin (solubility 81 μM vs 1.7 μM). Since macbecin I lacks a readily applicable chemical ‘handle’ for prodrug formation, our strategy centered on the reduction of the quinone moiety to the hydroquinone form, followed by masking the phenolic groups through either an ester or carbamate linkage. Addition of ionisable groups on these side chains improved water solubility dramatically. For example, one compound from this chemical series, BC274, has a solubility of >20 mM in 5% glucose, 10-fold higher than that of 17-DMAG. It is stable at pH <6.5 but converts to macbecin II and then macbecin I at physiological pH, for example in plasma or blood with a t_1/2 of 45-50 min. The conversion is not enzyme-dependent and therefore devoid of potential issues from metabolic polymorphism. BC274 showed a significantly improved MTTD over macbecin I. When given at 60 mg/kg to mice bearing DU145 prostate tumor xenograft, BC274 produced potent inhibition in tumor growth (T/C 30% at day 24 vs T/C 36% by 50 mg/kg 17-AAG at day 24). The significantly improved water-solubility coupled with its potent in vivo efficacy makes BC274 a candidate for further development.